The present invention relates to devices which measure noise present in radio frequency (RF) signals. More specifically, the present invention relates to noise measurement test systems for making phase noise and frequency noise measurements of devices that amplify or modify an RF signal.
Amplifiers are devices that increase the gain of a carrier signal. Amplifiers play a key role in many electronic systems today. Parameters such as gain, gain flatness, compression point, intermodulation and others have always been important to optimizing the performance of an amplifier. In addition, new parameters of an amplifier have become important to maintain the performance of an overall system. Unfortunately, in addition to amplifying the carrier signal, amplifiers typically introduce RF energy into an original RF signal in the form of intermodulation and sideband noises, including thermal noise, shot noise and flicker noise. This noise is typically random and is referred to as additive and residual phase noise and frequency noise. In addition, spurious noise signals can be generated by an amplifier. These consist of discreet signals appearing as distinct components called xe2x80x9cspursxe2x80x9d which can be related to power line and/or vibration modulation. It is important that this noise be minimized to the greatest extent possible. Since frequency noise is a function of phase noise, these noise components will be collectively referred to herein as phase noise.
The presence of phase noise in RF signal sources is a concern in several applications including applications related to analog and digital communications such as code division multiple access (CDMA) and time division multiple access (TDMA) cellular communication systems. The European Global System for Mobile communications (GSM) has promulgated detailed standards which define the operating requirements for both mobile and base station transmitters because the radio communication system as a whole will work properly only if each component operates within precise limits. Mobile transmitters and base stations must transmit outgoing RF signals with sufficient power, sufficient fidelity to maintain call quality and without transmitting excess power into frequency channels and time slots allocated to others.
For communications equipment particularly, it is important to minimize spurs and distortion products even when they appear below the amplitude level of the signal produced by an amplifier. This is important because the distortion or noise from numerous communication modules in a communications system tend to statistically add, thereby raising the level of noise in the overall system. Accordingly, it has now become important to measure the phase noise levels of an amplifier even below the level of the signal produced by the amplifier.
Phase noise is also of great concern in radar equipment, especially Doppler radar equipment which determines the velocity of a moving target by measuring the shifts in frequencies caused by return echos from a transmitted signal. The return echo signal is typically amplified for measurement. Unfortunately, a very large noise floor, up to 40 dB, has been observed near the carrier frequency. Such high background noise caused by the amplifiers of the radar equipment can result in degradation in target detection sensitivity and prevention of proper operation of land based and airborne active array radar. In some instances, it has been found that the introduction of phase noise caused by the amplifiers can partially or even totally mask the echo signal.
The most straight forward and least expensive technique for measuring the phase noise of an amplifier is to input a signal of known frequency into the amplifier and connect the output to a spectrum analyzer. However, it is difficult to measure the phase noise which is close in frequency to the carrier signal. In addition, using this technique, it is impossible to measure the noise caused by the amplifier which is below the amplitude of the amplifier output signal. In order to minimize the amount of noise generated by a particular system, it is highly advantageous to be able to measure that noise. As a result, there has been a continuing need for equipment which can make phase noise measurements.
Two approaches predominate for making phase noise measurements of an RF signal. The first system is a noise measurement test apparatus that uses a waveguide delay line descriminator. For example, U.S. Pat. No. 5,608,331 issued to Newberg et al. discloses a test system for making phase noise and amplitude noise measurements of microwave signals using a waveguide, coax and fiberoptic delay lines. The delay line descriminator uses the RF input from a unit under test (UUT) to generate a reference signal via the delay line for phase noise evaluation. The signal from the unit under test is split into first and second paths and combined again at a mixer which places the respective signals 90xc2x0 out of phase (in phase quadrature). Where the test system introduces very low noise or substantially no noise, the mixer outputs demodulated phase noise which can be measured by a sweeping spectrum analyzer.
The second conventional approach for making phase noise measurements makes use of the combination of noise from two phase lock RF sources. A low noise source is provided which provides a carrier signal to a unit under test, typically an amplifier. The low noise source also outputs a second low noise signal, at the same frequency as the carrier signal, which is combined with the carrier signal from the amplifier in a mixer. Using a phase shifter, the mixer places the two signals in phase quadrature.
Assuming no significant noise in the test setup, the mixer output signal represents the noise of the unit under test which can be measured by a sweeping spectrum analyzer.
Unfortunately, measurement of modulation and wide band noise measurements using prior art systems is expensive, difficult and time consuming. At wide offsets, such as 600 kHz, these measurements require high dynamic range which has historically been expensive. For these reasons, wide band noise measurements are typically only performed on a sample basis. Even conducting noise measurements on a sample basis is extremely time consuming as this technique requires a series of separate measurements to be performed requiring a lot of retuning of the test equipment. For example, utilizing the delay line technique described above, one must adjust phase shifters, attenuators and additional amplifiers at several frequency assignments across a broad bandwidth. Similarly, the above-described ultra low noise source test system typically requires making manual adjustments of a low noise source, a phase shifter, an additional amplifier and a buffer for each frequency offset across a broad bandwidth. The manual adjustment of each of these units usually takes ten minutes or more for each test measurement. Moreover, to ensure test accuracy, a large number of test samples must be taken with the increase in the number of sample measurements resulting in a decrease in the standard deviation in error of the noise measurements of the unit under test. Simply, sufficient readings must be taken to verify correct operation of the unit under test.
Unfortunately, performing such tests is both expensive and time consuming. Moreover, the prior art test systems are very expensive, typically costing between $100,000 and $200,000. In addition, these systems must be purchased piecemeal, requiring the separate purchase of amplifiers, low noise sources, signal attenuators, phase shifters, mixers and sweeping noise analyzers, which must be assembled to create a desired test system. The interconnection on each of these components creates additional regions for the introduction of phase noise into the system.
There is thus a need for a noise measurement test system which can accurately measure low level phase noise.
It would also be advantageous if a noise measurement test system were provided which had a high degree of reproducibility in the test measurements. To this end, it would be highly advantageous if a noise measurement test system were provided which eliminated a need for manual manipulation of the different components of the test system including the amplifiers, phase shifters, attenuators, low noise sources, etc. which typically must be adjusted for each noise measurement.
It would also be highly advantageous if a noise measurement test system could be provided in a single modular component which was lightweight and of compact size.
Briefly, in accordance with the invention, I provide an improved apparatus and method for automatically testing the phase noise of a unit under test. It is believed that my invention is particularly suitable for testing the phase noise of amplifiers even below the noise level of a carrier signal. The noise measurement system includes a variable low noise source for producing an adjustable low noise signal. The variable low noise source includes two outputs for outputting identical low noise signals, or is coupled to a splitter for splitting a single low noise signal into two identical low noise source signals. The first low noise signal is routed to a unit under test. The unit under test includes an input for receiving a first low noise carrier signal and an output for outputting a carrier signal. The UUT carrier signal is then routed through a variable amplifier where it is thereafter received by a mixer.
The second low noise signal output from the variable low noise source, is transmitted to a variable phase shifter which adjusts the phase of the second low noise signal to be 90xc2x0 out of phase (in phase quadrature) with the UUT signal passing through the variable amplifier. After being shifted in phase, the second low noise signal is received by the mixer at a second input port where it is combined with the UUT signal and output from a mixer output port. The variable amplifier and variable phase shifter are both adjusted so that the UUT signal and second noise signal are in phase quadrature and of matching amplitude such that if there were no noise in the system or in the UUT, a direct current (DC) is output from the mixer output port. However, assuming noise in the UUT and very low noise in the test system, the mixer outputs a signal representative of the noise of the unit under test. The signal output from the mixer, hereinafter referred to as a xe2x80x9cmeasurement test signalxe2x80x9d, is then sent to a variable low noise matching amplifier. The variable low noise matching amplifier both amplifies the measurement test signal and acts as a buffer. The matching variable amplifier is constructed to add very low noise so as not to interfere with the noise measurements of the unit under test and provides for amplification of the measurement test signal to enhance the ability to measure any noise in the unit under test.
After passing through the low noise matching amplifier the measurement test signal is received by an analog-to-digital converter (ADC) which converts the analog measurement test signal into digital data. The digital data is then transmitted to a processor for evaluation prior to sending the measurement test signal to a spectrum analyzer. The spectrum analyzer uses standard, windowed, fast or discreet fourier transforms that accurately measures the noise spectrum of the measurement test signal. These fourier transformers are known to those skilled in the art and will not be discussed in detail herein.
The processor is connected by a plurality of control lines to the variable amplifier, variable low noise source, variable phase shifter and the variable low noise matching amplifier. The processor sets levels and makes adjustments to the amplifier, low noise source, phase shifter and matching amplifier to obtain initial calibration and maintain optimum system sensitivity for measuring the phase noise of a unit under test. The processor performs these automated control functions to adjust the variable low noise source, amplifier, phase shifter and matching amplifier which are normally done manually. For example, to measure the noise of a unit under test, several test measurements of the unit under test must be made with the low noise source producing a carrier signal at different offset frequencies. Prior art systems require that the low noise source first be manually adjusted. The phase shifter must then be manually adjusted to ensure that the signals received by the mixer are in phase quadrature. The amplifier must also be adjusted to ensure that the signals received by the mixer are of the same amplitude. Moreover, the matching amplifier must be adjusted to ensure proper impendence between the mixer and analog-to-digital converter. These manual adjustments typically take ten minutes or more. The processor of the present invention provides for automatic adjustments of these components which typically can be accomplished in less than one minute. In addition, the processor of the present invention provides for the creation and automation of an entire test program, or protocol. A unit under test can be tested, xe2x80x9csteppedxe2x80x9d across a predetermined bandwidth by preprogramming the test system to conduct numerous test measurements of the UUT at different low noise source offset frequencies. In addition, the processor can be preprogrammed to conduct a sufficient number of test samples at each frequency offset to ensure that the noise level of the unit under test falls within acceptable levels.
To control the measurement test system, the signal processor takes the digitized output from the ADC to both calibrate the system and to ensure that the amplifier, low noise source and phase shifter are set to correct levels. More particularly, the output from the ADC enables the processor to determine whether the low noise source is providing a carrier signal at a correct frequency. By evaluating the output from the ADC, the processor can determine that the phase shifter is properly maintaining the signals received by the mixer in phase quadrature. Similarly, any failure by the variable amplifier to maintain a proper level of amplification of the UUT signal as received by the mixer can be corrected by the processor. If any of these components are not functioning optimally, the processor makes required adjustments to ensure proper noise testing of the unit under test by the measurement test signal.
In a preferred embodiment, the variable low noise source of the present invention outputs both a variable carrier signal and a variable calibration signal. The calibration signal is a very low level noise sideband having a precisely known magnitude relative to the magnitude of the carrier signal, typically about 60 dB below the amplitude of the carrier signal. The calibration signal enables calibration of the test measurement system. This calibration signal is not eliminated by phase quadrature in the mixer and would thus appear on the spectrum analyzer. Since the calibration signal has a known magnitude, the displayed height of the phase noise can be compared to the displayed height of the calibration signal. Any noise caused by the unit under test can be compared with calibration signal to quantitatively measure the phase noise of the unit under test.
In addition, the present invention provides for quantitatively calibrating the system at multiple levels by adjusting the magnitude of the calibration signal. More particularly, it is preferred that when the measurement test system is calibrated, that the calibration signal be provided sequentially at different magnitudes. For example, the calibration signal is first provided a xe2x88x9210 db relative to the carrier signal. Thereafter, the calibration signal is provided at xe2x88x9220 db, xe2x88x9230 dB, xe2x88x9240 dB, xe2x88x9250 dB and xe2x88x9260 dB relative to the carrier signal. The processor then graphically displays each of these calibration levels on the spectrum analyzer so as to provide a greater measure of accuracy in determining the actual phase noise created by the unit under test.
In an additional preferred embodiment, the processor controls the variable low noise source to produce multiple calibration signals which are offset in frequency during the test system calibration procedure. In other words, the present invention provides for calibrating the system in frequency and in amplitude each time the system is calibrated. For example, in addition to calibrating the system by producing calibration signals at xe2x88x9210 db through xe2x88x9260 db relative to the carrier signal, during calibration, the low noise source produces several additional calibration signals which are offset in frequency relative to the carrier signal. The processor then graphically displays each of these calibration levels on the spectrum analyzer so as to provide a greater measure of accuracy in determining the actual frequency of spurs or distortion products created by the unit under test. This information can be useful for determining the actual cause of any noise produced by the unit under test.
It is thus an object of the present invention to provide an improved noise measurement test system for measuring the RF noise of a unit under test.
It is an additional object of the present invention to provide an automated noise measurement test system which provides for automatic adjustments of the components of the system which can typically be accomplished in less than one minute. In addition, it is an object of the present invention to provide for an automated noise measurement test system which can create and implement an entire automated test program to desired requirements.
It is still another object of the present invention to provide an automated noise measurement test system which can calibrate the system at several decibel levels relative to the carrier signal and at several offset frequencies relative to the carrier signal each time the system is calibrated.
These and other further advantages of the present invention will be appreciated by those skilled in the art upon reading the following detailed description with reference to the attached drawings.